The present invention relates to a device for controlling ignition timing in internal combustion engines, and, more particularly, a device for controlling the vacuum advancing of ignition timing.
An internal combustion engine generally incorporates a vacuum advancer which advances ignition timing in accordance with the intensity of the intake vacuum in view of the fact that the combustion speed of fuel-air mixture in a cylinder chamber lowers as the intake vacuum increases. Such a vacuum advancer generally comprises a diaphragm means and an ignition timing advancing mechanism adapted to be actuated by the diaphragm means so that it advances ignition timing as the vacuum supplied to the diaphragm means increases. The diaphragm means is generally operated by a vacuum taken out from a port provided in an intake passage means such as a carburetor having a throttle valve, wherein the port is and located so as to be at the upstream side of the throttle valve when the throttle valve is fully closed and at the downstream side of the throttle valve when the throttle valve is slightly opened. By employing such a particular intake vaccum take-out port (advancer port), vacuum advancing of ignition timing is not effected when the engine is idling with the throttle valve being fully closed, while vacuum advancing of ignition timing is only actually effected when the throttle valve has been opened beyond a predetermined small opening, whereby a stabilized idling operation is ensured while a desirable vacuum advancing of ignition timing is effected in low load operating conditions. On the other hand, if the vacuum advancer is operated by an intake vacuum taken out from a port absolutely located at the downstream side of the throttle valve, an unduly large vacuum advancing of ignition timing is effected in idling operation thereby causing rough idling.
However, the fact that the aforementioned advancer port is generally employed for vacuum advancing does not mean that no advancing of ignition timing should be effected in idling operation. On the contrary, a moderate advancing of ignition timing in idling operation increases output power of the engine per unit amount of fuel thereby making it possible to reduce fuel consumption in idling. This is particularly effective in internal combustion engines for modern automobiles incorporating exhaust gas purification system in which the engine rotational speed in idling operation is made higher when compared with conventional engines.